1. Field of the Invention
The present invention relates to an image forming method and apparatus for forming multi-tone images.
2. Description of the Related Art
In image forming apparatus such as page printers, digital copying apparatus, and facsimiles, dot matrix methods such as a density pattern method and a dither pattern method have been used as the tone reproduction method for outputting multi-tone images onto paper.
The density pattern method is a method in which threshold values for the dots (elements) of an m*n matrix are associated with each single element of an original image so that a plurality of tone levels are represented by a plurality of mutually dissimilar dot area patterns.
The dither pattern method is a method in which threshold values for the dots of an m*n matrix are associated with each single picture element of an original image in one-to-one correspondence. The dither pattern method can be classified into Fattening types (dot-concentrated types) wherein dots are concentrated in the center of the matrix in halftone levels, and Bayer types (dot-dispersed types) wherein dots are dispersed throughout the entire matrix in halftone levels.
For the formation of hard copy images by the electrophotographic processes, when a Bayer type threshold value matrix is used, there is a tendency that a high degree of gamma recording characteristics result due to the effect of intensity distributions of exposure using laser beam or others. For this reason, a Fattening type threshold value matrix is used in usual cases.
On the other hand, when a Fattening type threshold value matrix is used, resolution is reduced when the matrix size is increased to increase the number of tone levels, and the number of tone levels is diminished when the matrix size is reduced to improve resolution.
By using these tone reproduction methods, multi-tone images have been output on the paper to a certain extent. However, there has been desired an image forming method and apparatus which are capable of outputting multi-tone images with high resolution and good reproducibility of tone level.